An expansion valve with a variable orifice area

ABSTRACT

An expansion valve ( 1 ) for a vapour compression system, the valve ( 1 ) comprising a first valve part ( 5 ) having an outlet orifice ( 7 ) and a piston ( 8 ) movable inside the outlet orifice ( 7 ) in response to a differential pressure across the expansion valve ( 1 ), controlling a fluid flow through the first valve part ( 5 ). A cross-sectional flow area of the outlet orifice ( 7 ) between a circumference at an inner surface of the outlet orifice ( 7 ) and a circumference at an outer surface of the piston ( 8 ) varies as a function of the position of the piston ( 8 ) relative to the outlet orifice ( 7 ). A first cross-sectional flow area is defined at a first differential pressure, and a second cross-sectional flow area is defined at a second differential pressure, where the first cross-sectional flow area is smaller than the second cross-sectional flow area, and the first differential pressure is lower than the second differential pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates by reference subject matter disclosed in the International Patent Application No. PCT/IB2014/062718 filed on Jun. 30, 2014 and European Patent Application No. 13175068 filed on Jul. 4, 2013.

TECHNICAL FIELD

The present invention relates to an expansion valve for a vapour compression system, such as a refrigeration system, an air condition system or a heat pump. In the expansion valve of the invention a cross-sectional flow area of an orifice is variable in response to variations in a differential pressure across the expansion valve. Furthermore, the expansion valve of the invention is adapted to counteract pressure transients in the vapour compression system following opening of the expansion valve.

BACKGROUND

Reduction of energy consumption of vapour compression systems, such as refrigeration systems, air condition systems or heat pumps is desired. Various efforts have been made to improve the components of vapour compression systems to reduce the energy consumption of the individual components, and of the vapour compression systems as such.

Furthermore, the expansion valve of a vapour compression system may advantageously be operable to control the amount of refrigerant being supplied to the evaporator to utilise the available cooling or heating capacity of the vapour compression system to the greatest possible extent.

To control an opening degree of an expansion valve of a vapour compression system in response to a differential pressure across the expansion valve is sometimes desirable. Control of the opening degree of the expansion valve occurring automatically when the differential pressure changes may also be desired.

U.S. Pat. No. 5,038,579, U.S. Pat. No. 5,052,192 and U.S. Pat. No. 5,002,089 all disclose an expansion valve comprising an elongated member extending into a metering port, the elongated member and the metering port cooperating to define a flow metering passage between the elongated member and the metering port. The elongated member is configured to vary the cross-sectional area of the flow metering passage in relation to the position of the elongated member to the flow metering port. Means are provided within the flow passage for controlling the position of the elongated member within the passage in response to the differential pressure across the expansion valve. The variations in the cross-sectional area of the flow metering passage defined by the elongated member are provided so that the cross-sectional area of the flow metering passage decreases as the differential pressure across the expansion valve increases.

WO 2012/072076 discloses an expansion valve for a vapour compression system, the expansion valve comprising a first valve member and a second valve member, one of the valve members being automatically movable in response to changes in a differential pressure across the expansion valve. The relative position of the first valve member and the second valve member determines an opening degree of the expansion valve. The variations in opening degree of the expansion valve are provided so that the opening degree decreases as the differential pressure across the expansion valve increases.

FR 2 661 977 discloses an expansion device comprising a movable piston comprising a metering port. The piston is movable inside a housing relative to an elongated member which is fixed relative to the housing, and which has a conical shape. Thereby the metering port and the elongated member cooperate to define a metering passage. A seal is required between the movable piston and the inner wall of the housing in order to provide a tight valve. This has the consequence that a certain minimum force must be applied to initiate movements of the piston, in order to overcome the friction force between the piston and the wall of the housing. Thereby it is difficult to move the piston in a precise manner, and it is not possible to adjust the flow through the valve in a precise manner. The defined metering passage is provided so that the size of the metering passage decreases as the differential pressure across the expansion device increases.

U.S. Pat. No. 4,341,090 discloses a combination variable orifice valve means and check valve means to control flow between an indoor coil and an outdoor coil. The valve comprises an orifice in a valve body, and a spring-biased valve plug movable relative to the orifice and cooperating with the orifice to vary the effective area of the orifice to control the flow of fluid through the orifice. The effective area decreases as the differential pressure increases.

SUMMARY

An object of embodiments of the invention may be to provide an expansion valve for a vapour compression system, the expansion valve being capable of counteracting transients in the vapour compression system after a compressor of the vapour compression system is switched on.

A further object of embodiments of the invention may be to provide an expansion valve which is suitable for supplying refrigerant to an evaporator of micro channel type.

As example, a micro channel heat exchanger may be installed as a condenser in a refrigeration system, the micro channel heat exchanger being a low refrigerant volume heat exchanger and a non-micro channel heat exchanger may be installed as evaporator in a refrigeration system, the non-micro channel heat exchanger being a high refrigerant volume heat exchanger compared to a micro channel heat exchanger technology, such as a fin-and-tube heat exchanger.

One or more compressors displaces refrigerant from the evaporator, and if the expansion valve is not reacting fast enough, the refrigeration system will store the refrigerant in the low refrigerant volume micro channel condenser. Refrigerant pressure at a high pressure side of the expansion valve increases quickly and triggers a low pressure switch installed in the refrigeration system.

Installing both a micro channel evaporator and a non-micro channel evaporator condenser, a combination not common within the technical field of refrigeration systems, the high pressure build up will not occur, as the condenser is not so sensitive to additional refrigerant. However, the low pressure switch of the system would probably trigger, if all refrigerant is removed from the evaporator and the expansion valve does not supply sufficient refrigerant fast enough.

The invention is applicable to different installations of heat exchangers, functioning either as a condenser or as an evaporator. One installation can be fin-and-tube condenser together with fin-and-tube evaporator. Another installation can be micro channel condenser together with fin-and-tube evaporator. Another installation can be micro channel condenser together with micro channel evaporator. Another installation can be fin-and-tube condenser together with micro channel evaporator.

Micro channel heat exchangers are heat exchangers in which (at least one) fluid flows in channels with small hydraulic cross-sectional area. Fluid flow in channels of small hydraulic cross-sectional area will predominantly be laminar in character. The correlation results in an increasing heat transfer coefficient when channel cross-sectional area decreases. For laminar flow in channels with small cross-sectional area, the heat transfer coefficient scales inversely with channel cross-sectional area, making micro channels desirable. Refrigerant viscosity determines the minimum practical channel cross-sectional area.

The present invention provides an expansion valve for a vapour compression system, the valve comprising:

-   -   a first valve part having an outlet orifice in relation to an         intended fluid flow direction through the first valve part, said         outlet orifice providing a forward fluid passage through the         first valve part, and     -   a piston having an inlet end and an outlet end in relation to an         intended fluid flow direction through the first valve part,         wherein the piston is movable inside the outlet orifice in         response to a differential pressure across the expansion valve,         controlling a fluid flow through the first valve part, via the         forward fluid passage through the first valve part,         wherein a cross-sectional flow area of the outlet orifice         between a circumference at an inner surface of the outlet         orifice and a circumference at an outer surface of the piston         varies as a function of the position of the piston relative to         the outlet orifice, and wherein a first cross sectional flow         area is defined at a first differential pressure, and a second         cross-sectional flow area is defined at a second differential         pressure, where the first cross-sectional flow area is smaller         than the second cross-sectional flow area, and the first         differential pressure is lower than the second differential         pressure.

In the present context the term ‘vapour compression system’ should be interpreted to mean any system in which a flow of fluid medium, such as refrigerant, circulates and is alternatingly compressed and expanded, providing either refrigeration or heating of a volume. The vapour compression system may be a refrigeration system, an air condition system, a heat pump, etc. The vapour compression system comprises a compressor, a heat rejecting heat exchanger, as example in the form of a condenser or a gas cooler, an expansion valve, and an evaporator, along a refrigerant path.

The expansion valve comprises a first valve part. The first valve part has an outlet orifice in relation to an intended fluid flow direction through the first valve part. The intended fluid flow direction is a direction which fluid normally flows in when passing through the expansion valve. The intended fluid flow direction may advantageously be the flow direction through the expansion valve during normal operation of the vapour compression system, and fluid flowing through the expansion valve is expanded when flowing in the intended fluid flow direction. Fluid flowing through the first valve part leaves the first valve part via the outlet orifice when flowing in the intended fluid flow direction.

The outlet orifice provides a forward fluid passage through the first valve part. In the present context, the term ‘forward’ means a flow direction corresponding to the intended fluid flow direction.

The expansion valve further comprises a piston having an inlet end and an outlet end in relation to the intended fluid flow direction through the first valve part. In the present context the term ‘piston’ means an elongated member movable inside the expansion valve. The inlet end of the piston is at a part of the expansion valve where fluid enters the expansion valve when flowing in the intended fluid flow direction, and the outlet end of the piston is at a part of the expansion valve where fluid leaves the expansion valve when flowing in the intended fluid flow direction.

The piston is movable inside the outlet orifice in response to a differential pressure across the expansion valve. In the present context the term ‘differential pressure’ means a pressure difference between a pressure level at an inlet side of the expansion valve and a pressure level at an outlet side of the expansion valve. When the pressure difference changes, the position of the piston inside the outlet orifice also changes, and the position of the piston inside the outlet orifice is determined by the differential pressure across the expansion valve. The position of the piston inside the outlet orifice determines a fluid flow through the outlet orifice, and thereby through the forward fluid passage through the first valve part. The fluid flow through the valve, via the forward fluid passage through the first valve part, is controlled in accordance with the differential pressure across the expansion valve.

It is an advantage that the piston, i.e. the elongated member, is movable in response to the differential pressure across the expansion valve, instead of the first valve part being movable and the piston being fixed, because thereby the movable part is not abutting the inner wall of the valve housing. Accordingly, there is no friction between the movable part and the fixed parts, and no friction force needs to be overcome in order to move the movable part. This allows the relative position between the piston and the outlet orifice, and thereby the fluid flow through the valve, to be controlled in a precise manner.

A cross-sectional flow area of the outlet orifice between a circumference at an inner surface of the outlet orifice and a circumference at an outer surface of the piston varies as a function of the position of the piston relative to the outlet orifice. The cross-sectional flow area of the outlet orifice, and the size of the opening through which fluid can flow through the first valve part, varies in response to variations in the differential pressure across the expansion valve.

A mass flow through the expansion valve also varies in response to variations in the differential pressure across the expansion valve. The cross-sectional flow area varies so that a first cross-sectional flow area is defined at a first differential pressure, and a second cross-sectional flow area is defined at a second differential pressure, where the first cross-sectional flow area is smaller than the second cross-sectional flow area, and the first differential pressure is lower than the second differential pressure. The cross-sectional flow area increases when the differential pressure is increased.

In some vapour compression systems the evaporator is a so-called micro channel heat exchanger being a heat exchanger in which the refrigerant passes through a plurality of parallel micro channels. The expansion valve must open relatively quickly when the compressor is switched on after an off cycle. If the expansion valve opens too slowly, the refrigerant supply to the evaporator is too small causing a significant increase in differential pressure across the expansion valve, and introducing undesirable transient effects. However, according to the invention, the increase in differential pressure causes the piston to be moved so that the part of the piston which is in the outlet orifice corresponds to the second cross-sectional flow area, and the cross-sectional flow area of the outlet orifice between a circumference at an inner surface of the orifice and a circumference at an outer surface of the piston is relatively large. The fluid flow through the expansion valve is increased, causing a decrease in differential pressure across the expansion valve and removal of the undesired transient effects. Accordingly, the expansion valve of the invention prevents transient effects, and is very suitable in vapour compression systems comprising a micro channel heat exchanger. This is due to the special design of the valve, where the cross-sectional flow area of the outlet orifice is increased when the differential pressure across the valve increases.

The performance of vapour compression systems, such as residential air conditioning units and heat pump units, must fulfil certain requirements to ensure that their energy consumption is kept at an acceptable level. Vapour compression systems are subjected to standardized tests which measure the performance of the vapour compression system under standard conditions. These tests are sometimes referred to as SEER tests (seasonal energy efficient ratio). For units having a single-speed compressor and a fixed-speed indoor fan two required test conditions exist in the cooling mode, referred to as the A test and the B test, and two optional tests, referred to as the C test and the D test, which need to be performed to evaluate the energy efficiency of the unit.

The purpose of the A test is to measure the nominal cooling capacity of the unit and the energy consumed during operation. The temperature corresponds to a hot climate condition (dry bulb temperatures: 35° C. at the outdoor and 26.7° C. at the indoor), and the ability to deliver cooling under these demanding conditions is monitored. Under conditions corresponding to a hot climate condition (dry bulb temperatures: 35° C. at the outdoor and 26.7° C. at the indoor), the differential pressure across the expansion valve is expected to be high.

The B test corresponds to a more averaged seasonal temperature (dry bulb temperatures: 27.8° C. at the outdoor and 26.7° C. at the indoor). Conditions corresponding to a more averaged seasonal temperature (dry bulb temperatures: 27.8° C. at the outdoor and 26.7° C. at the indoor) are more typically occurring than the conditions applied during the A test, and the result of the B test plays a more important part when the overall SEER rating is calculated for the unit. The unit performing in an energy efficient manner during the B test is important. Under conditions corresponding to a more averaged seasonal temperature (dry bulb temperatures: 27.8° C. at the outdoor and 26.7° C. at the indoor), the differential pressure across the expansion valve is expected to be lower than under the conditions prevailing during the A test.

To assess the cyclic performance of the unit, the two optional tests, the C test and the D test, may be performed. Basically, the ability of the unit to provide cooling after compressor start-up (D test) effectively is examined and the efficiency is related to the result recorded in the C test. The dry bulb temperatures at the outdoor and the indoor for these tests are the same as for the B test, but the humidity ratio of the indoor air needs to be sufficiently low to prevent that water condensates on the evaporator in the C and D tests.

The piston may have different cylindrical shapes stepwise along a longitudinal extension of the piston, the piston defining a first cross-sectional area along a first longitudinal extension and a second cross-sectional area along a second longitudinal extension, the first cross-sectional area being larger than the second cross-sectional area, and the first longitudinal extension may be in the outlet orifice at the first differential pressure and the second longitudinal extension may be in the outlet orifice at the second differential pressure. In the present context the term ‘cylindrical’ means that the piston has a cross-section which does not vary along a longitudinal direction of the piston. However, the cross-section, and the cross-sectional area, of the piston varies from one longitudinal extension to another.

According to this embodiment the varying cross-sectional flow areas of the outlet orifice are provided by a varying cross-sectional area of the piston, along a longitudinal direction defined by the piston. When the piston is moved inside the outlet orifice, along the longitudinal direction, and in response to variations in the differential pressure across the expansion valve, parts of the piston having varying cross-sectional areas will be in the outlet orifice, and varies the cross-sectional flow area defined between the piston and the outlet orifice. Also, because of the stepwise cylindrical shape of the piston, the cross-sectional flow area defined between the piston and the outlet orifice is substantially constant within two distinct differential pressure regions. Controlling the expansion valve in a very accurate manner is possible.

A third cross-sectional flow area of the outlet orifice between a circumference at an inner surface of the outlet orifice and a circumference at an outer surface of the piston may be defined at a third differential pressure, the third differential pressure being higher than the first differential pressure and lower than the second differential pressure, and the third cross-sectional flow area may be smaller than the first cross-sectional flow area.

According to this embodiment, the first cross-sectional flow area is defined between the piston and the outlet orifice at low differential pressures. At higher differential pressures, the third cross-sectional flow area, being a smaller cross-sectional flow area, is defined between the piston and the outlet orifice. At very high differential pressures, the second cross-sectional flow area, being a cross-sectional flow area which is larger than the first cross-sectional flow area and larger than the third cross-sectional flow area, is defined between the piston and the outlet orifice. Preferably, the cross-sectional flow area defined between the piston and the outlet orifice varies between the first cross-sectional flow area and the third cross-sectional flow area during normal operation of the expansion valve, and the second cross-sectional flow area is only defined at exceptionally high differential pressures, e.g. as described above. In this case, the first and third cross-sectional flow areas, respectively, may advantageously ensure that the refrigerant flow through the expansion valve is automatically adjusted to meet the energy efficiency requirements under the conditions of the B test and the A test, respectively, described above. The overall performance of the vapour compression system is improved.

The first valve part may comprise a valve seat at an outlet of the outlet orifice, and the piston may comprise a stop element, and a differential pressure below a predefined threshold value may cause the stop element of the piston to abut the valve seat of the first valve part, preventing fluid flow through the first valve part, via the forward fluid passage.

When the expansion valve of the invention is in a vapour compression system, the expansion valve expands refrigerant and controls the refrigerant supply to the evaporator. The expansion valve expanding refrigerant and controlling the refrigerant supply to the evaporator is done more or less automatically in accordance with the differential pressure across the expansion valve, as described above. If the compressor is switched off, the differential pressure across the expansion valve will immediately start decreasing, because the compressor no longer compresses refrigerant in the suction line, while the expansion valve continues to supply refrigerant to the evaporator. When the differential pressure across the expansion valve reaches the predefined threshold value, the stop element is, according to this embodiment, brought into abutment with the valve seat of the first valve part, thereby closing the valve. The consequence is that the refrigerant charge is kept in place, and decrease of the differential pressure across the expansion valve is delayed and is not decreasing below a saturation pressure level corresponding to the ambient temperature.

During an off period of the compressor forming part of the vapour compression system where the valve is arranged, the refrigerant is maintained at each side of the valve by the stop element. If a stop element is not provided, all refrigerant displaces to the coldest part in the system, and the differential pressure decreases quickly towards zero. At compressor start, the compressor needs to displace the refrigerant from low pressure side of the valve to high pressure side of the valve, before the refrigerant in the system is balanced and can cool down the ambient again. The stop element keeps the refrigerant in place.

Even when a stop element is provided, during a period of time the differential pressure will decrease, because a liquid part of the refrigerant will continue to evaporate, because of temperature difference between ambient temperature and temperature of refrigerant in the system, until a thermodynamic equilibrium is reached, and the refrigerant pressure in the system will be at the saturation pressure level corresponding to the ambient temperature. The period of time for achieving equilibrium depends on the mass of liquid evaporating in the separate sections of the system.

Maintaining the refrigerant in place, by providing a stop element, results in differential pressure across the valve being maintained for a longer period of time, although differential pressure is decreasing because of evaporation of the liquid part of the refrigerant, and results in the period of time being longer for a thermodynamic equilibrium to be reached. Differential pressure decreasing to zero may not be the reached during an off period of the compressor, if an off period of the compressor, before the compressor is stated again after the off period, is a limited period of time.

As example, if during operation of the compressor a differential pressure across the valve is 10 bar, after an off period of the compressor of, as example, 24 minutes, a differential pressure across the valve may be 3 bar for a valve according to the invention being provided with the stop element, compared to a differential pressure across the valve being zero bar for a valve not being provided with a stop element.

When the compressor is started again at a later point in time, the differential pressure across the expansion valve is at a relatively high level. Spending energy on establishing a required differential pressure across the expansion valve each time the compressor is switched from an off position to an on position is not necessary. The total energy consumption of the vapour compression system can be minimised. The stop element of the expansion valve allows the vapour compression system to perform well in the D test described above.

The piston may be moved in a direction towards a position where the stop element of the piston abuts the valve seat of the first valve part when the differential pressure across the expansion valve decreases, and the piston may be moved in a direction away from a position where the stop element of the piston abuts the valve seat of the first valve part when the differential pressure across the expansion valve increases. According to this embodiment, a part of the piston corresponding to the first cross-sectional flow area defined between the piston and the outlet orifice may be closer to the outlet end of the piston than a part of the piston corresponding to the second cross-sectional flow area defined between the piston and the outlet orifice.

The expansion valve may also comprise mechanical forcing means arranged to force the piston towards a position in which the stop element of the piston in brought into abutment with the valve seat of the first valve part. According to this embodiment, the differential pressure must work against the force applied by the forcing means, to open the expansion valve.

A differential pressure below a predefined threshold value causes the stop element of the piston to abut the valve seat of the first valve part, preventing fluid flow through the first valve part, via the forward fluid passage. The predefined threshold value is a differential pressure applying a force to the piston. The force applied to the piston exactly balances the force applied by the forcing means. At higher differential pressures, the force applied to the piston by the differential pressure exceeds the force applied to the piston by the forcing means. The resulting force on the piston will cause the piston to be moved so that the stop element is moved out of abutment with the valve seat. Similarly, at lower differential pressures, the force applied to the piston by the forcing means exceeds the force applied to the piston by the differential pressure. The resulting force on the piston will push the stop element into abutment with the valve seat.

The mechanical forcing means forcing the piston towards a position in which the stop element is brought into abutment with the valve seat is an advantage, because the stop element is brought reliably and quickly into abutment with the valve seat when the compressor is switched off.

The mechanical forcing means may comprise a compressible spring, said compressible spring being positioned between the inlet end of the piston and a spring contact surface of the first valve part. According to this embodiment the compressible spring pushes the piston towards the position where the stop element abuts the valve seat. As an alternative, other kinds of mechanical forcing means, such as a resilient member, a torsion spring, etc. may be used.

The stop element may be an elastomeric element fixed to the outlet end of the piston, and may be forming an enlarged head of the piston, said elastomeric element having a cross-sectional area being larger than the cross-sectional area of the valve seat, and said elastomeric element having an outer circumference being larger than an inner circumference of the valve seat.

The expansion valve may further comprise a valve housing,

-   -   wherein the first valve part is movable inside the valve         housing,     -   wherein a first position of the first valve part inside the         valve housing allows a forward fluid flow through the expansion         valve and prevents a reverse fluid flow through the expansion         valve, and     -   wherein a second position of the first valve part inside the         valve housing allows a reverse fluid flow through the expansion         valve and prevents a forward fluid flow through the expansion         valve.

According to this embodiment, the expansion valve is a dual flow valve, meaning that components of the valve may be switched between a position in which the components allow a forward fluid flow through valve, and a position in which the components allow a reverse fluid flow through the valve. When the first valve part is in the first position, the valve operates as an expansion valve, allowing a forward fluid flow through the valve, in the manner described above, while expanding the fluid.

When the first valve part is in the second position, a forward fluid flow through the valve is prevented, and instead a reverse fluid flow through the valve is allowed. The reverse fluid flow may advantageously be substantially unrestricted, and fluid flowing through the valve in the reverse direction is preferably not expanded. If a stop element is mounted on the piston, the stop element may abut the valve seat and prevent forward fluid flow through the expansion valve, when the first valve part is in the second position. A forward fluid flow through the valve is prevented because of the stop element abutting the valve seat.

The valve according to this embodiment may advantageously be used in vapour compression systems selectively operated in an air condition mode or in a heat pump mode. In vapour compression systems selectively operated in an air condition mode or in a heat pump mode, ability to reverse the flow direction of the fluid flow of the system is necessary. The valve must be able to provide the required expansion of refrigerant in one mode, while allowing an unrestricted fluid flow to pass in the other mode. The valve being able to provide the required expansion of refrigerant in one mode, while allowing an unrestricted fluid flow to pass in the other mode is obtained by the piston being movable inside a first valve part, which is in turn movable inside a valve housing.

The first valve part may also have an inlet orifice in relation to an intended fluid flow direction through the first valve part, said inlet orifice providing a rearward fluid passage through the first valve part. According to this embodiment, fluid flowing through the expansion valve enters the first valve part via the inlet orifice and leaves the first valve part via the outlet orifice.

A cross-sectional flow area of the inlet orifice between a circumference at an inner surface of the orifice and a circumference at an outer surface of the piston may be constant as a function of the position of the piston relative to the inlet orifice. If the cross-sectional flow area of the outlet orifice varies in response to the position of the piston, the fluid flow through the valve will be limited by the cross-sectional flow area of the outlet orifice, rather than by the cross-sectional flow area of the inlet orifice.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

FIGS. 1-3 are cross-sectional views of an expansion valve according to a first embodiment of the invention, and

FIGS. 4-8 are cross sectional views of a first valve part for an expansion valve according to a second embodiment of the invention.

DETAILED DESCRIPTION

FIGS. 1-3 are cross sectional views of an expansion valve 1 according to a first embodiment of the invention. The expansion valve 1 comprises a valve housing 2, a fluid inlet 3 and four fluid outlets 4, three of which are visible.

A first valve part 5 is movable inside the valve housing 2. The first valve part 5 has an inlet orifice 6 and an outlet orifice 7. A piston 8 is movable inside the first valve part 5. The piston 8 has an inlet end 8 a and an outlet end 8 b. A stop element 9 is mounted at the outlet end 8 b of the piston 8. The stop element 9 may be an elastomeric element.

A compressible spring 10 is mounted inside the first valve part 5 between the inlet end 8 a of the piston 8 and a spring contact surface 11. The compressible spring 10 forces the piston 8 in a direction towards a position where the stop element 9 abuts a valve seat 12 at the outlet orifice 7 of the first valve part 5. Furthermore, the piston 8 is movable in response to a differential pressure across the expansion valve 1.

FIG. 1 shows the first valve part 5 and the piston 8 in a position in which a forward fluid flow through the expansion valve 1 is allowed. The first valve part 5 is in abutment with the valve housing 2, via seal 13, the seal 13 preferably made of Teflon®. The piston 8 is moved in a direction to the left in the Figure, and the stop element 9 is moved out of abutment with the valve seat 12. A fluid flow through the expansion valve 1 from the fluid inlet 3 to the fluid outlets 4, via the inlet orifice 6 and the outlet orifice 7, is allowed, as illustrated by the arrow. As the fluid passes through the outlet orifice 7, the fluid is expanded, the valve 1 operating as an expansion valve when the first valve element 5 is in the position shown in FIG. 1.

FIG. 2 shows the first valve part 5 and the piston 8 in a closed position, where fluid flow through the valve is prevented. The first valve part 5 is still in abutment with the valve housing 2, preventing a reverse fluid flow through the expansion valve 1. The stop element 9 is in abutment with the valve seat 12 of the first valve part 5, preventing a forward fluid flow through the expansion valve 1.

FIG. 3 shows the first valve part 5 and the piston 8 in a position in which a reverse fluid flow through the expansion valve 1 is allowed. The first valve part 5 is moved to the right in the Figure, moving the first valve part 5 out of abutment with the valve housing 2. The stop element 9 is in abutment with the valve seat 12, preventing a forward fluid flow through the first valve part 5, as described above. A reverse fluid flow through the expansion valve 1 from the fluid outlets 4 to the fluid inlet 3, via a flow channel defined between the valve housing 2 and the first valve part 5, is allowed, as illustrated by the arrow. The reverse fluid flow through the valve 1 is substantially unrestricted. Accordingly, when a reverse fluid flow through the expansion valve 1 is allowed, the fluid is not expanded when passing through the valve 1.

The expansion valve 1 of FIGS. 1-3 is capable of selectively allowing fluid flow through the valve 1 in a forward direction, as illustrated in FIG. 1, or in a reverse direction, as illustrated in FIG. 3. The expansion valve 1 is suitable for use in a vapour compression system which operates as a combined air condition unit and heat pump unit. In combined air condition units and heat pump units reverse fluid flow through the system is needed to allow heat exchangers of the system to operate as a condenser or as an evaporator, depending on whether the system is operating as an air condition unit or the system is operating as a heat pump unit. It is an advantage that reverse fluid flow through the system can be obtained by the first valve part 5 being movable inside the valve housing 2.

FIGS. 4-8 are cross-sectional views of a first valve part 5 for an expansion valve according to a second embodiment of the invention. FIG. 4 shows the entire first valve part 5. The first valve part 5 comprises an inlet orifice 6 and an outlet orifice 7, defining a forward fluid flow passage through the first valve part 5.

A piston 8 having an inlet end 8 a and an outlet end 8 b is movable inside the first valve part 5, the piston 8 extending through the inlet orifice 6 and the outlet orifice 7.

A stop element 9 is at the outlet end 8 b of the piston 8, e.g. in the form of an elastomeric element. When the stop element 9 is in abutment with a valve seat 12 at the outlet orifice 7 of the first valve part 5, a forward fluid flow through the forward fluid flow passage of the first valve part 5 is prevented. When the stop element 9 is out of abutment with the valve seat 12, a forward fluid flow through the inlet orifice 6 and the outlet orifice 7 is allowed. Accordingly, a forward fluid flow through the first valve part 5 is determined by the position of the piston 8 relative to the outlet orifice 7.

A compressible spring 10 is mounted inside the first valve part 5 between the inlet end 8 a of the piston 8 and a spring contact surface 11. The compressible spring 10 forces the piston 8 in a direction towards a position where the stop element 9 abuts a valve seat 12 at the outlet orifice 7 of the first valve part 5. Furthermore, the piston 8 is movable in response to a differential pressure across the expansion valve. The forward fluid flow through the first valve part 5 is determined by the differential pressure across the expansion valve. Since the compressible spring 10 forces the piston 8 in a direction which brings the stop element 9 into abutment with the valve seat 12, the stop element 9 will be brought into abutment with the valve seat 12 at a differential pressure below a predefined threshold value, corresponding to the force applied by the compressible spring 10.

The part of the piston 8 which extends through the outlet orifice 7 has different cylindrical shapes stepwise along a longitudinal extension of the piston 8. The cross-sectional area of the piston 8 in a first region is larger than the cross-sectional area of the piston 8 in a second region, the first region being closer to the outlet end 8 b of the piston 8 than the second region. In a third, intermediate, region, arranged between the first region and the second region of the piston 8, the cross-sectional area of the piston 8 is larger than the cross-sectional area of the piston 8 in the first region. In each of the regions the cross-sectional area of the piston 8 is substantially constant.

When the part of the piston 8 corresponding to the second region is in the outlet orifice 7, a cross-sectional flow area defined between the piston 8 and the outlet orifice 7 is larger than a cross-sectional flow area defined between the piston 8 and the outlet orifice 7 when the part of the piston 8 corresponding to the first region is in the outlet orifice 7, and when the part of the piston 8 corresponding to the first region is in the outlet orifice 7, a cross-sectional flow area defined between the piston 8 and the outlet orifice 7 is larger than a cross-sectional flow area defined between the piston 8 and the outlet orifice 7 when the part of the piston 8 corresponding to the third, intermediate, region is in the outlet orifice 7.

FIGS. 5-8 show a detail of the first valve part 5 of FIG. 4. In FIG. 5 the stop element 9 abuts the valve seat 12. A forward fluid flow through the first valve part 5 is prevented, i.e. the expansion valve is in a closed position. In the situation illustrated in FIG. 5 the differential pressure across the valve is below the predefined threshold value.

In FIG. 6 the stop element 9 has been moved out of abutment with the valve seat 12. A forward fluid flow through the first valve part 5, via the forward fluid passage defined by the inlet orifice 6 and the outlet orifice 7, is allowed, and the valve is in an open position. In the situation illustrated in FIG. 6 the differential pressure across the valve is above the predefined threshold value. However, the differential pressure is relatively close to the predefined threshold value, and the piston 8 has only been moved a small distance away from the position in which the stop element 9 abuts the valve seat 12. The part of the piston 8 which is in the outlet orifice 7 corresponds to the first region, and a relatively large cross-sectional flow area is defined between the piston 8 and the outlet orifice 7.

In FIG. 7 the piston 8 has been moved further in the direction which moves the stop element 9 away from the valve seat 12. The differential pressure across the valve in the situation illustrated in FIG. 7 is higher than the differential pressure across the valve in the situation illustrated in FIG. 6. Therefore, the part of the piston 8 which is in the outlet orifice 7 corresponds to the third, intermediate, region, and the cross-sectional area defined between the piston 8 and the outlet orifice 7 is smaller than is the case in the situation illustrated in FIG. 6.

In FIG. 8 the piston 8 has been moved even further in the direction which moves the stop element 9 away from the valve seat 12. The situation illustrated in FIG. 8 corresponds to a very high differential pressure across the valve. Therefore, the part of the piston 8 which is in the outlet orifice 7 corresponds to the second region, and the cross-sectional area defined between the piston 8 and the outlet orifice 7 is larger than is the case in the situations illustrated in FIGS. 6 and 7.

In the case that the expansion valve supplies refrigerant to an evaporator of micro channel type, there is a risk that the valve opens too slowly following a start-up of the compressor, and the refrigerant supply to the evaporator is insufficient. This causes a significant increase in the differential pressure across the expansion valve, and introduces undesired transients in the system. The increase in differential pressure across the expansion valve causes the piston 8 to be moved to a position where the part of the piston 8 which is in the outlet orifice 7 corresponds to the second region, i.e. the situation illustrated in FIG. 8. A very large cross-sectional flow area is defined between the piston 8 and the outlet orifice 7, resulting in a large fluid flow through the expansion valve. Accordingly, a sufficient refrigerant supply to the evaporator is ensured, eliminating the undesired transients and decreasing the differential pressure across the expansion valve.

In the first valve part 5 of FIGS. 4-8 the cross-sectional area defined between the piston 8 and the outlet orifice 7 varies according to the position of the piston 8 relative to the outlet orifice 7, and according to the differential pressure across the valve. Furthermore, because of the stepwise cylindrical shape of the piston 8, the cross-sectional flow area defined between the piston 8 and the outlet orifice 7 is substantially constant within three distinct differential pressure regions allowing the fluid flow through the valve to be controlled in an accurate manner.

The first valve part 5 of FIGS. 4-8 may advantageously be movable inside a valve housing in the manner described above with reference to FIGS. 1-3.

While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. An expansion valve for a vapour compression system, the valve comprising: a first valve part having an outlet orifice in relation to an intended fluid flow direction through the first valve part, said outlet orifice providing a forward fluid passage through the first valve part, and a piston having an inlet end and an outlet end in relation to an intended fluid flow direction through the first valve part, wherein the piston is movable inside the outlet orifice in response to a differential pressure across the expansion valve, controlling a fluid flow through the first valve part, via the forward fluid passage through the first valve part, wherein a cross-sectional flow area of the outlet orifice between a circumference at an inner surface of the outlet orifice and a circumference at an outer surface of the piston varies as a function of the position of the piston relative to the outlet orifice, and wherein a first cross-sectional flow area is defined at a first differential pressure, and a second cross-sectional flow area is defined at a second differential pressure, where the first cross-sectional flow area is smaller than the second cross-sectional flow area, and the first differential pressure is lower than the second differential pressure.
 2. The expansion valve according to claim 1, wherein the piston has different cylindrical shapes stepwise along a longitudinal extension of the piston, the piston defining a first cross-sectional area along a first longitudinal extension and a second cross-sectional area along a second longitudinal extension, the first cross-sectional area being larger than the second cross-sectional area, and wherein the first longitudinal extension is in the outlet orifice at the first differential pressure and the second longitudinal extension is in the outlet orifice at the second differential pressure.
 3. The expansion valve according to claim 1, wherein a third cross-sectional flow area of the outlet orifice between a circumference at an inner surface of the outlet orifice and a circumference at an outer surface of the piston is defined at a third differential pressure, the third differential pressure being higher than the first differential pressure and lower than the second differential pressure, and wherein the third cross-sectional flow area is smaller than the first cross-sectional flow area.
 4. The expansion valve according to claim 1, wherein the first valve part comprises a valve seat at an outlet of the outlet orifice, and the piston comprises a stop element, and wherein a differential pressure below a predefined threshold value causes the stop element of the piston to abut the valve seat of the first valve part, preventing fluid flow through the first valve part, via the forward fluid passage.
 5. The expansion valve according to claim 4, wherein the piston is moved in a direction towards a position where the stop element of the piston abuts the valve seat of the first valve part when the differential pressure across the expansion valve decreases, and in a direction away from a position where the stop element of the piston abuts the valve seat of the first valve part when the differential pressure across the expansion valve increases.
 6. The expansion valve according to claim 4, further comprising mechanical forcing means to force the piston towards a position in which the stop element of the piston in brought into abutment with the valve seat of the first valve part.
 7. The expansion valve according to claim 6, wherein the mechanical forcing means comprises a compressible spring, said compressible spring being positioned between the inlet end of the piston and a spring contact surface of the first valve part.
 8. The expansion valve according to claim 4, wherein the stop element is an elastomeric element fixed to the outlet end of the piston, and is forming an enlarged head of the piston, said elastomeric element having a cross-sectional area being larger than the cross-sectional area of the valve seat, and said elastomeric element having an outer circumference being larger than an inner circumference of the valve seat.
 9. The expansion valve according to claim 1, further comprising a valve housing, wherein the first valve part is movable inside the valve housing, wherein a first position of the first valve part inside the valve housing allows a forward fluid flow through the expansion valve and prevents a reverse fluid flow through the expansion valve, and wherein a second position of the first valve part inside the valve housing allows a reverse fluid flow through the expansion valve and prevents a forward fluid flow through the expansion valve.
 10. The expansion valve according to claim 1, wherein the first valve part further has an inlet orifice in relation to an intended fluid flow direction through the first valve part, said inlet orifice providing a rearward fluid passage through the first valve part.
 11. The expansion valve according to claim 10, wherein a cross-sectional flow area of the inlet orifice between a circumference at an inner surface of the orifice and a circumference at an outer surface of the piston is constant as a function of the position of the piston relative to the inlet orifice.
 12. The expansion valve according to claim 2, wherein a third cross-sectional flow area of the outlet orifice between a circumference at an inner surface of the outlet orifice and a circumference at an outer surface of the piston is defined at a third differential pressure, the third differential pressure being higher than the first differential pressure and lower than the second differential pressure, and wherein the third cross-sectional flow area is smaller than the first cross-sectional flow area.
 13. The expansion valve according to claim 2, wherein the first valve part comprises a valve seat at an outlet of the outlet orifice, and the piston comprises a stop element, and wherein a differential pressure below a predefined threshold value causes the stop element of the piston to abut the valve seat of the first valve part, preventing fluid flow through the first valve part, via the forward fluid passage.
 14. The expansion valve according to claim 3, wherein the first valve part comprises a valve seat at an outlet of the outlet orifice, and the piston comprises a stop element, and wherein a differential pressure below a predefined threshold value causes the stop element of the piston to abut the valve seat of the first valve part, preventing fluid flow through the first valve part, via the forward fluid passage.
 15. The expansion valve according to claim 5, further comprising mechanical forcing means to force the piston towards a position in which the stop element of the piston in brought into abutment with the valve seat of the first valve part.
 16. The expansion valve according to claim 5, wherein the stop element is an elastomeric element fixed to the outlet end of the piston, and is forming an enlarged head of the piston, said elastomeric element having a cross-sectional area being larger than the cross-sectional area of the valve seat, and said elastomeric element having an outer circumference being larger than an inner circumference of the valve seat.
 17. The expansion valve according to claim 6, wherein the stop element is an elastomeric element fixed to the outlet end of the piston, and is forming an enlarged head of the piston, said elastomeric element having a cross-sectional area being larger than the cross-sectional area of the valve seat, and said elastomeric element having an outer circumference being larger than an inner circumference of the valve seat.
 18. The expansion valve according to claim 7, wherein the stop element is an elastomeric element fixed to the outlet end of the piston, and is forming an enlarged head of the piston, said elastomeric element having a cross-sectional area being larger than the cross-sectional area of the valve seat, and said elastomeric element having an outer circumference being larger than an inner circumference of the valve seat.
 19. The expansion valve according to claim 2, further comprising a valve housing, wherein the first valve part is movable inside the valve housing, wherein a first position of the first valve part inside the valve housing allows a forward fluid flow through the expansion valve and prevents a reverse fluid flow through the expansion valve, and wherein a second position of the first valve part inside the valve housing allows a reverse fluid flow through the expansion valve and prevents a forward fluid flow through the expansion valve.
 20. The expansion valve according to claim 3, further comprising a valve housing, wherein the first valve part is movable inside the valve housing, wherein a first position of the first valve part inside the valve housing allows a forward fluid flow through the expansion valve and prevents a reverse fluid flow through the expansion valve, and wherein a second position of the first valve part inside the valve housing allows a reverse fluid flow through the expansion valve and prevents a forward fluid flow through the expansion valve. 